Git (GitHub) commit at past date

git commit --allow-empty --date="Sat Nov 14 14:00 2015 +0100" -m '2 Dec commit'

It will create empty commit with provided date

While the question "How do I make a Git commit in the past?" explains how to amend the commit author date:

git commit --amend --no-edit --date="Fri Nov 6 20:00:00 2015 -0600"

Run that after a commit to amend the last commit with the timestamp noted.
The --no-edit will leave the message as-is.

The OP asks:

That question does not specify the GitHub result... would it work in the same way?

Yes: multiple projects exist allowing you to generate and push commits "done in the past", in order to update your contribution chart.

See for instance contribution.io, github-contribution, or gitgardener.

All you need to do, is to push those amended commit on the master branch of your GitHub repo, as I mention here.

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